The present invention relates to a method for preparing ester end-capped polyalkylene ether. In particular, it relates to a method for preparing ester end-capped polyalkylene ether by ring-opening polymerization of cyclic ether in the presence of a solid acid of a mixed oxide of group III and group IV elements as a catalyst and a mixture of acid and acid anhydride as a molecular weight modifier.
Ester end-capped polyalkylene ethers are very important intermediates for industrial applications which can be converted to polyether glycols by hydrolysis or alcoholysis. Polyether glycols are used as a starting material for plastic elastomers such as polyurethanes, and spandex fibers. Due to their superelasticity, superior chemical resistance, and air permeability, they have been extensively used in the manufacture of flexible, air ventilating clothes; panty hose; impact resistant motor parts; adhesives; paints; artificial organs; and artificial blood vessels.
Conventional manufacturing process for polyether glycols involves polymerizing cyclic ether into corresponding acid anion end-capped polyethers by the ring-opening process in the presence of a strong liquid acid as the catalyst, followed by subjecting them to hydrolysis or alcoholysis to form hydroxy group end-capped polyether glycols. For example, U.S. Pat. No. 4,510,333 and EP 0167292 A1 disclose a method in which tetrahydrofurans are polymerized into flurosulfonic radical end-capped polyethers by the ring-opening process in the presence of fluorosulfonic acid as a catalyst. U.S. Pat. No. 3,712,430 and Japan Patent No. Sho 45-13940 disclose the same method but fuming sulfuric acid and perchloric acid are respectively used as catalysts instead of flurosulfonic acid. One disadvantage of these methods are that the molecular weight distribution of the resulting polyether is rather broad, the number average molecular weight ranges from 500 to several ten thousands and the molecular distribution (Mw/Mn) is much greater than 2. Another disadvantage of this method is that the liquid acid catalyst used can not be recycled for reuse because it is very difficult to separate from the product prior to the hydrolysis or alcoholysis reactions. This leads to the consumption of a large amount of energy. Furthermore, the corrosion of reaction vessels, and the need of sequential treatment of waste acid require a great deal of labour and cost.
U.S. Pat. Nos. 4,153,786 and 4,202,964 and German Patent GB 2025995 disclose a new process which involves carrying out ring-opening polymerization in the presence of a cationic ion exchange resin bearing --SO.sub.3 H groups as a catalyst and an acetic anhydride as a precursor. However, since the water content of the catalyst must be strictly limited, the raw material tetrahydrofurans must be predehydrated to contain less than 100 ppm of water. Furthermore, as the cationic ion exchange resin used is normally poor-heat resistant, there are difficulties when a regeneration is desired.
U.S. Pat. Nos. 4,564,671, 4,728,722 and 4,803,299 disclose a process for the preparation of polytetramethyl ether acetate(PTMEA), which uses a acidic clay or a zeolite as catalyst and acetic anhydride as promoter. The disadvantage to this process is that the molecular weight distribution of the resulting PTMEA is relatively broad, and the Mw/Mn is about 3.0.
U.S. Pat. No. 4,568,775, Japan Patent Nos. Sho 63-30931, 63-30930, and laid-open Patent Nos. Sho 61-268787 60-158218 disclose a process for directly producing polyether glycols by using a heteropoly-acid as a catalyst. The process can synthesize polyether glycols in a single step, however, as the heteropoly-acid catalyst is readily soluble, it will remain in and stain the final products, and therefore requires of further treatment of the final product. Furthermore, the yield of this process is rather low, usually less than 10%.